Linear predictive coding (LPC) is a conventional technique known in the art for representing a speech signal or other sound signal. This technique allows for the digital transmission of speech at medium to low bit rates, such as 1000 to 9600 bits per second. According to the LPC technique, a speech waveform is divided into consecutive frames having equal durations. The duration of each frame is typically on the order of ten to twenty milliseconds. Each frame includes a gain value related to the energy of the frame, a pitch value representing the fundamental frequency of the voice or sound, and a set of parameters representing the speech spectrum. Different types of non-energy parameters may be used in representing a frame, with the pitch value and a plurality of digital filter reflection coefficients being standard.
Linear predictive coding separates the parameters pertaining to the spectral envelope from those related to the vocal tract excitation or pitch for each speech frame. This separation of data in turn allows modifications in pitch, speed and energy, and permits the novel effect described below.